Reductive Transformation of MnO2 Controls Thallium Remobilization: Differential Effects of Layered and Tunneled Structures

Mn oxides play a critical role in Tl scavenging and accumulation in the epibiotic environment. However, the effect of Mn oxide reduction in the Mn/Fe cycle on Tl mobilization is not clear. Herein, the influence of Mn oxide configuration, oxygen environment, and degree of reduction on MnO₂ transformation and associated Tl species distribution is investigated. In oxic environments, both typical δ-MnO₂ and α-MnO₂ structures (i.e., layered and tunneled, respectively) can immobilize Tl(I) for a long time. In mild-to-moderate reducing anoxic environments, the drastic reductive transformation of δ-MnO₂ results in Tl binding, mainly in an exchangeable form. In highly reducing environments, δ-MnO₂ or α-MnO₂ is converted to Manganite, resulting in the release of more Tl. Tl-L_III edge X-ray absorption spectroscopy indicates that oxidized Tl(III) (54-62%) is converted to structural Tl(I) (67-80%) and bound to interlayer/tunnel centers during the reductive transformation of MnO₂, which enhances Tl exchange in δ-MnO₂ and leads to Tl immobilization in α-MnO₂. Our results show that anoxic Tl(I)-/Mn(II)-/Fe(II)-induced MnO₂ transformation can enhance Tl mobilization, and tunneled MnO₂ may have a more sustainable Tl immobilization potential than layered MnO₂, which improves the general understanding of the geochemical behavior of Tl in different Mn-related reducing environments.